GNU bug report logs - #52873
expr unexpected syntax error

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Message #24 received at 52873-done <at> debbugs.gnu.org (full text, mbox):

From: Martin Rixham <martin.rixham <at> equalexperts.com>
To: Paul Eggert <eggert <at> cs.ucla.edu>
Cc: 52873-done <at> debbugs.gnu.org, Davide Brini <dave_br <at> gmx.com>
Subject: Re: bug#52873: expr unexpected syntax error
Date: Wed, 29 Dec 2021 22:07:25 +0000

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ok I appreciate the explanation.

On Wed, 29 Dec 2021 at 20:58, Paul Eggert <eggert <at> cs.ucla.edu> wrote:

> On 12/29/21 12:01, Martin Rixham wrote:
> > What nonsense. I want to parse source code. ')' is not an uncommon line
> of
> > source code. It should work.
>
> Unfortunately, you're asking for what is in general impossible. If the
> left argument of ':' could be any string, then the grammar for 'expr'
> would be ambiguous. Consider the following shell command:
>
> expr '(' : ')'
>
> This outputs ':' because it evaluates the parenthesized string ':'; but
> if the operands of ':' could be any strings it could also be interpreted
> as matching '(' against ')', which means it should output the same thing
> as 'expr a : b', namely '0'.
>
> Of course this means 'expr' was poorly designed in the 1970s, but we're
> stuck with that design now (it's standardized by POSIX), portable code
> must deal with this poor design, and for compatibility reasons it's
> better for GNU expr to support the design, poor as it is.
>
> These days there are much better ways than 'expr' to parse code. For
> example, if you want to count the number of characters in a shell
> variable v, you can use this shell command:
>
> nv=${#v}
>
> This works even if v=')', whereas this:
>
> nv=$(expr "$v" : '.*')
>
> has the bug that you mentioned, plus it's harder to read and it's less
> efficient.
>

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